Cooling method and apparatus

ABSTRACT

Apparatus for adaptive cooling comprising a first component having at least one aperture extending therethrough with a sacrificial component positioned within the at least one aperture. The first component is operable at a maximum duty temperature and the sacrificial component has a melting or sublimation point below the maximum duty temperature of the first component. The sacrificial component defines an effective aperture the size of which may be increased if, in use, the sacrificial component is subjected to a temperature between the melting or sublimation point of the sacrificial component and the maximum duty temperature of the first component.

BACKGROUND

This is a Division of application Ser. No. 11/488,071, filed Jul. 18,2006. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

This invention concerns a method and apparatus for adaptive cooling.

In particular it concerns a method and apparatus for adaptive cooling ofa duct wall, such as the wall of a combustor, afterburner or exhaustduct of a gas turbine engine.

Gas turbine engines for air, land or marine vehicles, and for energygeneration typically comprise, in axial flow series, a compressor, acombustor, a turbine and occasionally, for certain applications, anafterburner.

Air enters the engine and is compressed within the compressor beforefuel is added to the air flow and ignited within the combustion chamber.The hot gas drives a turbine, which powers the compressor. Excess energyis extracted as work from the turbine or used to generate thrust in theaircraft application. To generate additional thrust it is possible toinject and ignite further fuel in an afterburner. Ultimately the hot gasis expelled to the environment via an exhaust duct.

The temperature of the air entering the turbine has a bearing on theefficiency of the turbine. To this end it is desirable to run theturbine at as high a temperature as possible—often in excess of 1000° C.To enable operation at these high temperatures it is necessary to coolthe walls of the combustion chamber, the wall of the afterburner(commonly referred to as a heatshield) or the wall of the exhaust ductto prevent their damage.

One known method is film cooling. Metering, or “effusion cooling”, holesthat extend through the thickness of the components wall allowrelatively cool cooling air to enter the combustor, afterburner and/orexhaust duct. The air forms a protective film on the inner surface ofthe wall. The film is continually replenished with fresh, cold air.

To ensure adequate cooling for the entire surface it is necessary toprovide a multiplicity of holes with diameters between 0.4 and 0.7 mm.

Due to the large number of holes and their small size, significantvariations in total area and hence flow and cooling effectiveness canarise. Consequently, great attention is paid to the drilling of theholes, and in some cases flow tests are carried out as part of theinspection process. This can add a large overhead in terms of both timeand cost.

Periodic features in the combustor can also give rise to variation inthe heat load applied to the wall. It is difficult, and hence notpractical, to vary hole size or pitching in a circumferential direction.To counter this, the holes are formed such that the regions of highestheat load are effectively cooled and this results in excessive coolingfor areas presented with a lower heat load.

The local excess of cooling exacerbates thermally induced stresses inthe component and reduces the overall efficiency of the gas turbine bydiverting air which could otherwise be used for combustion.

The configuration of the cooling holes is fixed and cannot respond toconditions arising from faults or partial failures of other components.These conditions can manifest as significantly different heat-load tothat for which the component was validated. For example, the engine maybe subjected to a bird-strike or ingest other debris during operationwhich can obstruct some cooling features and detrimentally adjust theheat distribution within an effusion cooled component.

SUMMARY

It is an object of the present invention to seek to provide an improvedmethod and apparatus to seek to address these and other problems.

According to the present invention there is provided apparatus foradaptive cooling comprising

-   a first component having at least one aperture extending    therethrough, the first component operable at a maximum duty    temperature;-   a sacrificial component having a melting or sublimation point below    the maximum duty temperature of the first component and positioned    within the at least one aperture;-   wherein the sacrificial component defines an effective aperture the    size of which may be increased if, in use, the sacrificial component    is subjected to a temperature between the melting or sublimation    point of the sacrificial component and the maximum duty temperature    of the first component.

The first component may have opposing faces, the first face beingadapted to lie adjacent a hot area having a temperature T1, the secondface being adapted to lie adjacent a cold area having a temperature T2,wherein T1>T2.

The effective apertures are arranged to permit a flow of coolant fromthe cold area to the hot area.

The apertures or effective apertures are perpendicular to the firstface, or may be angled with respect to the first face.

Preferably the sacrificial component is a coating, which may be formedby electroplating, dipping, spraying, precipitation from gaseous phasereactants, painting, condensation.

Preferably the first component is the wall of a combustion chamber,which may be the combustion chamber of a gas turbine, or afterburner ofa gas turbine. Alternatively the first component may be the wall of anexhaust duct for a gas turbine engine or other internal combustionengine.

According to a second aspect of the present invention there is provideda method of adaptive cooling, the method comprising the steps

-   providing a first component having at least one aperture extending    therethrough, the first component operable at a maximum duty    temperature;-   providing a sacrificial component having a melting or sublimation    point below the maximum duty temperature of the first component and    positioned within the at least one aperture, thereby defining an    effective aperture;-   applying heat such that the temperature of the sacrificial component    is raised to a temperature between its melting or sublimation point    and the maximum duty temperature of the first component wherein the    effective aperture increases in size.

Preferably the method further comprises the step of passing a flow ofcoolant through the effective aperture. Preferably, as the effectiveaperture increases in size the temperature of the sacrificial componentis reduced by the flow of coolant to a temperature below the melting orsublimation point of the sacrificial component.

The first component may be the wall of a combustion chamber, which maybe a gas turbine combustion chamber, or afterburner of a gas turbine.Alternatively the first component may be the wall of an exhaust duct fora gas turbine engine or other internal combustion engine. Preferably thecoolant is air.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 depicts a gas turbine afterburner heatshield according to thepresent invention;

FIG. 2 is an enlarged image of the encircled area A of FIG. 1;

FIG. 3A is an alternative embodiment to that shown in FIG. 2, where asacrificial component is provided as an annular insert;

FIG. 3B is a plan view of the embodiment presented in FIG. 3A;

FIG. 4A is an alternative embodiment of the present invention, where asacrificial component is provided as a grooved insert;

FIG. 4B is a plan view of the embodiment presented in FIG. 4A;

FIG. 5A is an alternative embodiment of the present invention where asacrificial component is provided as a perforated insert;

FIG. 5B is a plan view of the embodiment presented in FIG. 5A;

FIG. 6A is an alternative embodiment of the present invention where asacrificial component is provided as a porous insert; and

FIG. 6B is a plan view of the embodiment presented in FIG. 6A.

FIGS. 1 to 6B show a heatshield wall 12 mounted to a casing 8 in a gasturbine engine. The heatshield wall 12 has a first face 18 and a secondface 20. The wall also has apertures 16 that allow air (depicted asarrow 10) to pass from a relatively cold area 4 (area adjacent to thesecond face 20) into the combustion chamber 2 (area adjacent to thefirst face 18).

DETAILED DESCRIPTION OF EMBODIMENTS

The apertures 16 are produced to a uniform size of greater diameter thanthat calculated to supply an adequate flow of air for cooling the wallat the normal operating temperature of the heatshield. The apertures maybe formed by ablation with a laser.

The apertures 16 are arranged at an angle to the face of the walladjacent the combustion chamber to allow the flow of air through theaperture to provide a robust film on that face. The heatshield wallwhich defines the combustor 2 has a maximum duty temperature of about950° C.

Turning now to FIG. 2 specifically, a sacrificial component 14 in theform of a coating is applied to the apertures 16 at the entrance, exit,and/or within the bore of the aperture 16 to provide an effectiveaperture that passes a lesser amount of air to that passed by theuncoated aperture.

The coating 14 has a melting or sublimation point below the maximum dutytemperature of the heatshield wall. The coating 14 forms an obstructionthat reduces the flow area of the aperture and consequently the massflow rate of coolant fluid.

If, in use, the local temperature rises to a temperature at or above themelting or sublimation point of the coating 14 then a proportion of thecoating 14 is removed. The effective aperture and the flow area for thecooling flow is increased in size and this permits an increased coolingflow.

The increased cooling flow reduces the local temperature and maintainsthe temperature of the heatshield wall within the limits of its maximumduty temperature.

The coating 14 is preferably applied through an electroplating process.Beneficially, the sharp edges of the aperture 16 results in preferentialdeposition at the entrance and exit. The effective aperture area isachieved with a minimum of coating material 14 and this provides both aweight and cost benefit.

Where silver is used as the sacrificial component material the effectiveaperture size will begin to increase when the local temperature rises toaround 950° C., this being the melting point of the silver. Theeffective aperture size may be modified at higher or lower temperaturesdepending on the choice of coating material. For example, if copper orgold, having melting points of 1080° and 1060° C. respectively, could beused to modify the aperture size at different temperatures to that ofsilver.

For applications other than combustion chambers or afterburners lead ortin may be used, which have melting points of 330° and 230° C.respectively. Such applications include, but are not limited to, anexhaust duct of a gas turbine engine or a duct of any device which isexposed to temperatures approaching the duty temperature of the ductmaterial.

Sacrificial materials other than pure metals may also be used. These canbe deposited using methods of application other than electroplating,such as dipping in molten material or plasma spray. Other types ofrefractory material could be deposited by precipitation from gaseousphase reactants. For low activation temperature, a polymer or paint-typematerial could be used; the polymer could be produced by reactiondirectly on the substrate; paint could be applied conventionally.

Alternatively, and as show in FIGS. 3A to 6B, the sacrificial componentmay be provided as a solid insert which is sized to fit in the apertureprovided in the first component and manufactured from at least one ofthe materials described herein. The insert 14, that is to say thesacrificial component 14, is configured such that when inserted into theaperture 16 of the first component 12, the effective cross sectionalarea of the flow path through the aperture 16 is partially reduced. Forexample, in the case of an aperture 14 with a substantially circularcross-section the sacrificial component may be an annular sleeve asshown in FIGS. 3A and 3B. Alternatively it may be a cylindrical plugwith axially extending grooves on the periphery of the plug which definea plurality of flow paths as shown in FIGS. 4A and 4B. In anotherembodiment the sacrificial component is provided with a plurality ofperforations as shown in FIGS. 5A and 5B. In another embodiment thesacrificial component is a porous plug as shown in FIGS. 6A and 6B.

The proportion of flow increase achievable by the invention can be setby choice of hole size and the thickness of the sacrificial component,whether the sacrificial component is provided as a coating or an insert.

The diameters of cooling holes for high and low pressure combustionsystems lie in the range Ø 0.5-2.5 mm. For a diameter of Ø 0.7, and aninitial sacrificial component thickness of 0.15 mm, a maximum increasein the effective aperture and flow area by a factor of 3.1 would resultif the entire sacrificial component is removed.

If desired, the sacrificial component may be removed and a new coatingor insert provided during servicing to restore the combustor wall to itsoriginal state.

It will be appreciated that the invention offers a number of advantages.In particular, the wall of the first component can be protected fromevents in service that increase the local temperature to above themaximum duty temperature of the wall. The wall can be protected even ifassociated components fail or there is a loss of coolant pressure

Additionally, the wall may be provided with an optimised cooling flow atthe start of its life, which remains optimised during the life of thecomponent as the cooling flow is automatically adjusted to respond tonon-uniformity in the heat-load to the component.

In some circumstances the precise temperature profiles within acomponent, such as the combustor, afterburner or exhaust duct of a gasturbine engine, are not easily predicted, the invention allows thecooling flow of the component to be automatically adjusted.

Further, the invention allows the relaxation of manufacturingtolerances, as normally the nominal size of cooling holes is chosen tobe larger than required to ensure that the resultant cooling hole size,even on its minimum tolerance, is still adequate. The saving of coolingflow will give thermodynamic advantages to the engine cycle e.g. byminimising coolant frictional losses, reducing work required topressurise coolant, and in the case of an afterburner, releasing thisflow to take part in the combustion process (giving higher thrustboost).

The sacrificial component need not be as robust as the wall of the firstcomponent and may be selected for its melting/sublimation temperature.The first component provides rigidity and support for the sacrificialcomponent. Consequently, for low activation temperature, a polymer orpaint-type material could be used where the polymer could be produced byreaction directly on the substrate. Paint could be appliedconventionally.

Various modifications may be made without departing from the scope ofthe invention.

For example, the apertures in the first component may be formed using aprocess other than ablation, including conventional techniques such asdrilling, electro discharge machining, or as part of a casting process.Such processes can produce apertures of a lower tolerance which aremodified through the addition of the sacrificial component to define thesize of the effective aperture. The apertures can also be larger thanthose in current constructions because their size will be modifiedthrough the addition of the sacrificial component. Larger apertures aregenerally cheaper to produce than smaller apertures and consequently thecost of manufacture is reduced.

This system could be applied to any gas-turbine combustor or afterburneremploying film cooling methods such as normal effusion, angled effusion,machined rings etc.

1. A method of adaptive cooling, the method comprising the stepsproviding a first component having at least one aperture extendingtherethrough, the first component operable at a maximum dutytemperature; providing a sacrificial component having a melting orsublimation point below the maximum duty temperature of the firstcomponent and positioned within the at least one aperture, therebydefining an effective aperture; applying heat such that the temperatureof the sacrificial component is raised to a temperature between itsmelting or sublimation point and the maximum duty temperature of thefirst component wherein the effective aperture increases in size.
 2. Amethod of adaptive cooling according to claim 1, further comprising thestep of passing a flow of coolant through the effective aperture.
 3. Amethod according to claim 1, wherein as the effective aperture increasesin size the temperature of the sacrificial component is reduced by theflow of coolant to a temperature below the melting or sublimation pointof the sacrificial component.
 4. A method according to claim 1, whereinthe first component is a wall of a combustion chamber.
 5. A methodaccording to claim 4, wherein the combustion chamber is a gas turbinecombustion chamber.
 6. A method according to claim 4, wherein thecombustion chamber is a combustion chamber in an afterburner.
 7. Amethod according to claim 1 wherein the first component is a wall of anexhaust duct.
 8. A method according to claim 1, wherein the coolant isair.